1. Field of the Invention:
The invention relates in general to protective relay apparatus, and more specifically to protective relay apparatus for protecting a polyphase power line section.
2. Description of the Prior Art:
Certain phase comparison arrangements of the prior art develop local and remote square wave pulse trains from a three-phase network of sequence filters and a mixing transformer. Thus, a single square wave train is produced at each line end, and these pulse trains are phase compared to detect a fault in the protected power line section. Variations of this technique use two separate comparisons, one for the positive sequence, and one for the negative sequence. However, both approaches incorporate sequence networks, and thus both are vulnerable to abnormal frequencies and phase impedance imbalances, which may arise in series compensated transmission lines.
In order to overcome the complexities involved in protecting series compensated transmission lines, a new and improved phase isolated relaying system was developed, such as disclosed in U.S. Pat. No. 3,893,008, which is assigned to the same assignee as the present application. In this system, four separate phase comparison sub-systems are utilized, i.e., three phases and ground, and is thus referred to as segregated phase comparison relaying. This approach overcomes the three major problem area of series compensated lines, i.e., the hereinbefore mentioned abnormal frequencies and phase impedance imbalance, and also the problem of voltage reversal due to the negative reactance of the series capacitor. It is a high-speed system because of angle diversity between the phases, and also due to the elimination of sequence filters. It is inherently redundant because the four sub-systems back up each other. It has the usual advantages of current-only relaying, i.e., it is not responsive to system swings, it is not subject to mutual induction problems, it is unaffected by loss of potential, it relays correctly for zero-voltage, three-phase faults, and it is not affected by potential transients. The phase isolation makes it possible to provide both relay and circuit breaker independent pole protection. Any desired degree of pole tripping selectivity may be provided due to the inherent phase selectivity for all fault types.
The main disadvantage of the isolated phase approach is the requirement for four separate pilot signals and thus four comparison relay units at each end of the protected line. An alternate approach, such as disclosed in U.S. Pat. No. 3,963,964, which is also assigned to the same assignee as the present application, reduces the number of comparison units by phase comparing phasor differences in the current in any two phases, and phase comparing the phasor sum of the current in all three phases. It would also be possible to phase compare the phasor difference in any two phases, and the ground current. However, with either of these approaches, the ability to single pole trip is lost, as it is not possible to determine which pole, or poles, to trip.
It would be desirable to reduce the number of comparison units required, if this reduction can be made without losing the ability to determine which phase (or phases) is faulted.